The present invention relates to an automatic exposure device used in such image forming apparatuses as electrostatic copying machines and more particularly it relates to an automatic exposure device for image forming apparatuses wherein a document is irradiated by an exposure lamp and the reflected light from the document is condensed to form an image corresponding to the document image, said automatic exposure device being adapted to control the power of the exposure lamp accurately in accordance with the density of the document.
In an electrostatic copying machine, which is a kind of image forming apparatus, because of the necessity of obtaining copies having optimum image quality with respect to a wide variety of documents to be copied, it has been common practice for the operator to visually estimate the density of a document and accordingly manipulate a density adjusting knob installed on an operating section so as to change the exposure.
However, the above method relying on the operator's visual estimation is inefficient or such visual estimation varies with the individual, thus posing a problem that copies having optimum image quality cannot always be obtained.
Therefore, in recent years there has been employed a system for detecting the intensity of reflected light from a document and automatically control the exposure for the document by the resulting detection signal.
An electrostatic copying machine using this system has a detecting element adapted to detect the intensity of the reflected light from a document irradiated by an exposure lamp, and the position on the document at which the reflected light intensity is detected by the detecting element is located at a position spaced by an amount corresponding to the distance traveled by the document or exposure lamp during the rise response time of the exposure lamp to be controlled, that is, at a position upstream of the exposure region of the document as viewed in the direction of travel of the document (a position downstream as viewed in the direction of travel of the exposure lamp). And the applied voltage on the exposure lamp is controlled by the quantity of light (reflected light) received by said detecting element.
In such system, however, because of the reading of the document density, the quantity of light of the exposure lamp which should originally be maintained constant is varied in accordance with the output of the detecting element receiving the reflected light from the document, the following problem arises.
For example, suppose a case where a document 4 having a high density region 23 and a low density region 24 is to be copied, as shown in FIG. 6. In addition, the corresponding position on the document 4 being detected by the detecting element is shown by the reference character 21a, while the position corresponding to the exposure region of the document 4 is shown by the reference character 18a. First, in the state of the document 4 assuming the position of FIG. 6(a) as a result of exposure movement, the quantity of light incident on the detecting element, corresponding to the low density region 24 of the document 4, is high, as shown in FIG. 7(1), so that the voltage to be applied to the exposure lamp is controlled to be low, as shown in FIG. 7(2), and hence the output light quantity of the exposure lamp is also small, as shown in FIG. 7(3). Next, in the state of the document moved to the position of FIG. 6(b), the quantity of light incident on the detecting element is small correspondingly to the density of the high density region 23 of the document 4, so that the lamp voltage is controlled to be high. However, the output light quantity of the exposure lamp lags in response by an amount corresponding to the rise time. As the light quantity of the exposure lamp increases, so does the quantity of light incident on the detecting element, so that the lamp voltage is controlled to be low. When the lamp voltage is controlled to be low, the output light quantity of the lamp is also controlled to be low, but there is a fall time which lasts for the same period as said rise time. Thus, because of a lag in response between the detecting element and the exposure lamp due to the rise and fall times of the latter, the copy of the document 4, as shown in FIG. 8, is striped with alternating high and low density regions 23' and 24' corresponding to the changes in the output light quantity of the exposure lamp shown in FIG. 7(3). In addition, the quantity of light incident on the detecting element and the output light quantity of the exposure lamp, whose actual changes are analog, are represented in simplified form in FIG. 7 for convenience' sake.
As an approach to such problem, it may be easily conceived to radiate light of predetermined quantity, separately from the exposure lamp, toward the document 4, to receive the reflected light by a light detecting element to detect the density of the document to thereby control the power of the exposure lamp. With such arrangement, however, it would be necessary to protect the detecting element from the influence of the light from the exposure lamp, in which case, then, it would be unavoidable to arrange the detecting element so that it detects the density of a portion of the document which is sufficiently away from the region of the document being exposed to the light from the exposure lamp. Then, it would be impossible to control the power of the exposure lamp accurately in accordance with the density of the document.
An object of this invention is to provide a novel automatic exposure device for use with image forming apparatuses, which ensures that irrespective of the degree of density of the background of a document, the background of the document image is kept in a clear state free from fogs and fringes to thereby improve the quality of images.
An automatic exposure control device according to the present invention comprises lamp light quantity detecting means for detecting the light quantity of an exposure lamp, density detecting means which receives the reflected light from a region of a document located forwardly of a region of the document being exposed to detect the density of the firt region, reference signal producing means which uses as its input signal the output signal from said lamp light quantity detecting means to produce a reference signal, correcting means which uses as its inputs the output signal of the density detecting means and the reference signal to correct the output signal of the density detecting means correspondingly to an increase in the lamp light quantity, and lamp control means which uses as its input the output signal of the correcting means to control the conducting-angle of the exposure lamp.
The lamp light quantity detecting means, which is preferably one adapted to detect the quantity of light of the exposure lamp by directly receiving the light, may be one adapted to detect said quantity of light on the basis of the applied voltage on the exposure lamp.
The reference signal producing means may be one adapted to output the difference between the output signal and a preset predetermined signal. Alternatively, it may be one adapted to delay the output signal of the lamp light quantity detecting means by an amount corresponding to the response time of the exposure lamp and output the difference between said output signal and a preset predetermined signal.